1. Field of the Invention
The invention relates generally to the analysis of particles which are suspended in atmospheric air or other gases and more particularly to an acoustical method and apparatus for detecting, sizing and counting such particles.
2. Description of the Prior Art
It has been known for more than a decade that when airborne particles are passed at high velocities through a capillary tube and then suddenly projected into an expanded exit cavity, audible acoustical pulses are produced which, according to some authors, appear to be generated in the vicinity of the capillary exit. Each of the pulses so obtained is attributable to a single particle and consists of a decaying sinusoidal oscillation having a duration of from about 0.5 to 30 milliseconds depending upon the design and operational characteristics of the acoustical system employed.
Although the mechanism by which such audio pulses are produced has not yet been established, the possibility that the phenomenon might be used to size airborne particles has been investigated as evidenced by the report of G. Langer in the Journal of Colloid Science 20, 602-609 (1965).
Langer notes in this report that he applied the principle in the laboratory to count ice crystals in supercooled air streams and found that his sensor was capable of detecting particles down to 5 microns in size. He further notes that when an attempt was made to relate pulse amplitude to particle size, it was found that pulse amplitude was independent of particle size and density. Thus, pulse amplitude did not provide a means for screening particle size. Indeed, he states that "If the sensor could measure particle size, many other applications would be possible".
U.S. Pat. No. 3,434,335 was awarded to Langer for the device described above. The Acoustical Particle Detector and Method of the present invention uses a detecting unit similar to that of Langer but adds an important feature lacking in the Langer device directed to particle size differentiation by selection of the corresponding flow Reynolds number for the gas in the capillary section of the device.
In a subsequent study reported by Reist and Burgess in the March-April 1968 issue of the American Industrial Hygiene Association Journal, an experiment was conducted in which the same particle was repeatedly passed through an acoustic detector and it was found that the initial pulse amplitude would vary by more than 100% for a single particle. This finding was considered by the authors to present a major impediment to the use of this acoustic system for particle sizing.
A still further study on the application of the aforenoted acoustic phenomenon to the detection and counting of airborne particles is reported in a doctoral dissertation titled "Pulse Processing From Particle Detectors" by David R. Hemenway which was submitted to the Faculty of the University of North Carolina, Chapel Hill, North Carolina, in 1974 and is now on file in the library of that institution. This study was directed principally to improving the detection procedures of acoustical sensors.
From the foregoing, it is apparent that considerable attention has been given over the years to the development of a useful acoustical particle detector. However, despite these efforts and the progress resulting therefrom, the acoustical phenomenon has remained a laboratory curiosity. Practical application of the principle in the analysis of airborne particulate size has been restrained by the lack of a capability for differentiating particles on the basis of size. It is, therefore, a principal object of this invention to provide an acoustical method and apparatus for analyzing airborne particles wherein such particles can be both counted and sized.